The present invention relates generally to digital communications, and, more particularly, to methods and systems for demodulating a received signal.
Various approaches have been developed for demodulating a received signal. For a conventional (single user) receiver it is known to use single or uni-directional demodulation. Bi-directional demodulation has also been proposed to improve performance using multiple known fields within the received signal as described in U.S. Pat. Nos. 5,335,250 and 5,400,362. Furthermore, as described in U.S. Pat. No. 5,909,465, the performance of the bi-directional demodulation can be enhanced by performing a first demodulation pass, calculating figure of merit (quality) values related to this demodulation, choosing demodulation directions for each sub-block of unknown data based upon these figures of merit, and then performing a second demodulation pass.
Joint demodulation or interference cancellation may be used as an alternative to single user demodulation to handle co-channel interference in communication systems, such as time division multiple access (TDMA) systems. Joint demodulation may be used to detect two or more signals that are received over a common channel. For example, joint demodulation may be used to detect a desired signal from a received signal that includes an interfering signal. In joint demodulation, the desired signal and the interfering signal are typically jointly demodulated based on information concerning the desired signal and the interfering signal, so as to obtain a better estimate of the desired signal.
Two-user joint demodulation for ANSI IS-136 TDMA mobile terminals has been proposed for cancellation of a dominant interfering signal under the assumptions of a flat, slow fading downlink channel environment. By subtracting off the interfering signal, the desired signal's bit error rate may be improved. This occurs where the channel and symbol data corresponding to the interfering signal are not correlated with the desired signal, thereby allowing separation of the two signals. Joint demodulation may, therefore, rely upon the ability to generate channel estimates and perform symbol detection for both the desired signal and the interfering signal. Detection of the desired signal may be improved, therefore, due to improved detection and cancellation of the interfering signal.
Interference cancellation may also be performed by spatial discrimination of the interferer relative to the desired signal if multiple receive antennas are available. However, for systems such as the TDMA IS-136 system, it has been proposed to allow downlink power control on a time-slot basis. This may impact interference cancellation approaches as the time-slots are not necessarily slot-aligned. Thus, the interferer power level may change, or even disappear, during the desired signal's slot. In general, interference cancellation approaches will have some loss, relative to the conventional demodulation approaches, when no interferer is present. Even more dramatic performance losses may occur when the interferer changes its power level abruptly, as different quantities (such as channel estimates or impairment covariance matrices) are typically used during demodulation of the slot. Even if these estimates are updated adaptively, the adaptation rate is typically not fast enough to withstand a sudden step change in the interferer characteristics.